Dangerous water feature some plumber grouts (4)
I believe the answer is:
berg
'dangerous water feature' is the definition.
(as in an iceberg)
'some plumber grouts' is the wordplay.
'some' indicates a hidden word (some of the parts of the word are used).
'BERG' can be found hidden inside 'plumber grouts'.
(Other definitions for berg that I've seen before include "One who scored" , "Mountain; Alban --, composer" , "Alban ----, composer of the incomplete opera 'Lulu' (1935)" , "He composed" , "feature of icy waters" .)